I tried several different themes, including the "Turf" one used in the preview. At the moment I'm using the "Wood" one as the contrast on the white inbox listings is in no way easy on the eyes. Which is a shame, I quite liked the "Pebbles" theme.

Been trying to figure this out too. Just noticed on the bottom right side of the gmail page that there is a little link asking if you want to try the new look. I didn't see it at first because I had my gmail task list open. If there is something like gmail task list fixed to the bottom right side of your gmail page try getting rid of it. It might be covering up the option to try the new gmail.